The present invention is directed towards a novel hybrid thick film substrate and, more particulary, to one having a metal base with overlying layers of selectively screened and fired thick film materials to provide necessary electrical properties.
There are numerous electrical circuit designs which require that conductive connections be formed upon a base substrate. In the electrical industry, printed circuit boards are formed on a great variety of base materials, For the most part, the substrates are composed of material with a relatively low thermal conductivity such as epoxy-glass or ceramic. In the fabrication of printheads useful in thermal printing applications, ceramic substrates have been preferred because of their excellent thermal stability, reasonable costs, and good electrical properties. While these properties have proven satisfactory for many applications, they do not provide for optimum electrical and thermal performance for all systems, particularly for the thermal ink jet printing application. For this application, for example, the thermal conductivity requirements are generally more stringent than those afforded by the ceramic class of substrate materials. This is because of the temperature control standard required to maintain a heated and pulsed printhead at the required operating temperature. Another difficulty with the prior art substrates, particularly ceramic, is the difficulty of achieving close dimensional tolerances of the substrate.
The above disadvantages can be overcome by using a metal as the base of the substrate. A metal, for example, copper, is 20 times as conductive as alumina ceramic and enables either maximum cooling or constant elevated (or reduced temperature operation) a condition most desirable for thermal ink jet printing. A metal substrate can be blanked to close tolerances and is cost effective in high volume manufacturing operations. A further advantage of using metal in the substrate is that it can be used as a ground plane and for EMI shielding for those applications requiring such functions.
The problems associated with using metal as a substrate have been the difficulty in overcoating the metal substrate with the thick film layers necessary to enable the electrical components to be mounted on the substrate, and the implementation of suitable passivation techniques to prevent oxidation of the base metal.
There are some prior art examples of metal substrates used for various applications. U.S. Pat. No. 3,613,230 Griff discloses a method for fabricating circuitry which utilizes a pure copper sheet as a base plate with a dielectric layer (such as epoxy), and a copper foil layer laminated on the copper base plate. The copper foil layer is then etched to form a desired pattern and a second dielectric layer and copper foil layer are laminated over the first layer. U.S. Pat. No. 4,307,147 to Ohishi et al. discloses an electrically insulating substrate having good thermal conductivity, the substrate is used for the manufacture of a printed wiring board. The substrate may comprise a highly thermal conductive metal plate, such as aluminum, copper, silver or iron (see Col. 3, lines 28-30). A film composed of a dispersion of metal oxide particles is formed on the metal plate. U.S. Pat. No. 4,221,925 to Finley et al. discloses a printed circuit board having a metal substrate with a fired dielectric layer coated thereon. The resultant coated substrate is then passed through standard metallization processes to form a desired electrical circuit pattern.
While these prior art designs may be satisfactory for the circuits and functions disclosed therein, there are other applications which utilize more complex geometries and have more stringent operating requirements. The present invention is, therefore, directed to a novel hybrid thick film substrate which, in one described embodiment, provides the support for a thermal ink jet printhead. However, the invention is not limited to any such particle application, but is applicable to any system which requires high thermal conductivity and stability afforded by the substrate design of the present invention. More particularly, the invention is directed toward a highly thermally conductive substrate comprising a conductive metal base, a passivation layer overlying said metal base and at least one screened and fired dielectric layer overlying said passivation layer.